The subject matter discussed in the background section should not be assumed to be prior art merely as a result of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches.
The transmission and storage of computer data increasingly relies on the use of codecs (coder-decoders) to compress/decompress digital media files to reduce the file sizes to manageable sizes to optimize transmission bandwidth and memory use. Transform coding is a common type of data compression for data that reduces signal bandwidth through the elimination of certain information in the signal. Sub-band coding is a type of transform coding that breaks a signal into a number of different frequency bands and encodes each one independently as a first step in data compression for audio and video signals. Transform coding is typically lossy in that the output is of lower quality than the original input. Many present compressors fail to remedy problems associated with compression artifacts, which are noticeable distortion effects caused by the application of lossy data compression, such as pre-echo, warbling, or ringing in audio signals, or ghost images in video data.
Many sub-band audio codecs, such as MP3, can partition a frame of audio data into multiple (possibly overlapping) blocks in order to more accurately represent transient signals, which are signals that change abruptly in time. Such partitioning helps eliminate distortions caused by quantization that would otherwise spread over the entire frame, creating an artifact known as “pre-echo.” Pre-echo and similar effects are caused when distortion artifacts are audible before the temporal event that caused them. One solution to eliminate pre-echo artifacts is to partition the audio frames into a large number of relatively small blocks. When the bit rate is limited, however, all of the bits may be spent coding the transient (at least in some portions of the spectrum). This leaves no bits available for the surrounding blocks, and causes a “partial collapse” wherein none of the energy in one or more regions of the spectrum in one or more blocks is coded. This partial collapse leaves a hole in the band that can be just as audible as any pre-echo artifact. This problem is especially acute in codecs that utilize small blocks and encode multiple small blocks (e.g., up to eight blocks) at one time.
What is needed, therefore, is a system to detect and fill coding holes created by collapsed blocks that are not encoded due to lack of available bits, so as to avoid any partial collapse artifacts, while attempting to ensure that no pre-echo artifacts are introduced.